Power requirements for modern portable electronics are increasing very rapidly; e.g., devices having larger displays, Long Term Evolution (LTE) communication devices (radios, modems, etc.), multi-core processors, and so on. To maintain acceptable operating durations, such devices increasingly utilize batteries with higher capacity. In such systems, battery charging times tend to be very long when conventional power sources are used. The reasons include: (1) limited power capability (e.g., for a universal serial bus (USB), a 5V/1.8 A maximum); and (2) voltage headroom issues between input power source and battery. Furthermore, many readily available power sources (e.g., monitors, notebooks, etc.) cannot be utilized because of their high-voltage operation versus what the portable device can tolerate. Also, implementing a solution that requires the use of a secondary portable device connector (e.g., proprietary connector, wall adapter, etc.) significantly increases solution and consumer cost.
With battery capacities increasing, 5V input voltage does not provide enough voltage headroom to achieve sufficiently high charge currents due to cable, connector, printed circuit board (PCB), and charger impedances. Many batteries now have a float voltage of about 4.35V, which makes this issue worse, especially since the trend is toward the use of higher voltages. For example, a 2S (e.g., 2 cell) stack provides about 8.4V or 8.7V, thus requiring a voltage higher than 5V to charge efficiently.